Advances in microelectromechanical systems (MEMS) technology continues to increase the applicability of MEMS devices to a variety of technological endeavors. By way of example, MEMS integrated circuit devices can be found in inkjet printers, automobile airbag systems, automobile stability control systems, optical switches, pressure sensors, computer and television display systems, and optical switches. Further applications for MEMS devices continue to be developed. As used herein, MEMS device refers generally to a micro-scale or nano-scale apparatus having two or more parts adapted to move relative to one another, where the motion is based on any suitable interaction or combination of interactions, such as mechanical, thermal, electrical, magnetic, optical, and/or chemical interactions. MEMS devices include, without limitation, NEMS (nanoelectromechanical systems) devices, MOEMS (micro-opto-electromechanical systems) devices, and devices with analogous functionalities having alternative nomenclatures as may be currently or hereinafter used or adopted.
The operation of many MEMS devices involves actuation that closes a gap between two members such that their surfaces are brought into actual or imminent contact with each other. By way of example, a simple MEMS-based optical switch may comprise a cantilevered mirror element disposed above a substrate layer, such that a voltage applied between the mirror element and the substrate layer causes flexing of the mirror element toward the substrate by electrostatic attraction, whereby a light beam impinging upon the mirror element is redirected according to the applied voltage. In another example, large arrays of such mirror elements can be placed on a single chip and individually driven by separate voltages to form a DMD (deformable mirror device) for use in a computer or television display system.
Moveable MEMS elements, such as the above-described cantilevered mirror element, often have high surface area to volume ratios such that surface effects can become competitive with mass, inertia, and the various “intentional” forces in dictating the actual movement (or lack thereof that physically occurs. So-called stiction forces, for example, can cause a mirror element to stick to a substrate and prevent the mirror element from moving. Various mechanisms have been proposed to explain such adhesion, including solid bridging, liquid bridging, “cold welding,” Van der Waals forces, and hydrogen bonding. Often the stuck part can be separated with increased force, but sometimes a permanent bond is formed after the initial contact. In addition to degrading device performance or causing outright device failure, stiction issues can also underlie increased margin requirements in device design (e.g., building in a higher spring coefficient for a deformable member, increasing actuation/release voltages, etc.) which can increase device size, cost, and power requirements, while reducing device speed and efficiency. Other issues arise as would be apparent to one skilled in the art in view of the present disclosure.